Heat pipes use successive evaporation and condensation of a working fluid to transport thermal energy, or heat, from a heat source to a heat sink. Because most working fluids have a high heat of vaporization, heat pipes can transport large amounts of heat in a vaporized working fluid. Further, the heat can be transported over relatively small temperature differences between the heat source and heat sink. Heat pipes generally use capillary forces through a porous wick to return condensed working fluid, or condensate, from a heat pipe condenser section (where transported thermal energy is given up at the heat sink) to an evaporator section (where the thermal energy to be transported is absorbed from the heat source).
FIG. 1 shows a longitudinal cutaway view of a typical heat pipe 100 that includes a conventional wick. Heat pipe 100 is shown being shorter than is typical to show all elements in a single figure. The primary elements of heat pipe 100 are a hermetically sealed container 112, a wick 114 and an interior vapor space 116. Typically the wick 114 is composed of a porous metal with mean pore diameters of about 100 μm, or fine axial grooves of similar width and depth. Known heat pipes also generally incorporate sintered powder “bi-porous” metal wicks which have two different dominant pore sizes to promote both low liquid pressure drop and high capillary pumping pressures, typically 100 μm transport pores and 30 μm pumping pores. In addition to porous media wicks small axial grooves are also often in heat pipes.
To reveal details, one end cap for sealed container 112 is not shown. Saturated inside wick 114 is a liquid working fluid (or coolant) 118, which typically comprises ammonia, methanol, water, sodium, lithium, fluorinated hydrocarbons or other fluid selected for its high heat of vaporization and acceptable vaporization temperature and other transport properties in a preselected temperature range within which the heat pipe 100 will operate. Heat pipe 100 typically includes an evaporator section 120, an optional adiabatic section 122 and a condenser section 124.
In operation, the evaporator section 120 of the heat pipe is placed into thermal contact with a heat source 126 and the condenser section 124 is placed into thermal contact with a heat sink 128. As thermal energy from heat source 126 is supplied to evaporator section 120, liquid working fluid 118 impregnating the wick 114 absorbs the thermal energy and begins to vaporize, undergoing a phase change from liquid to vapor. The vapor pressure of the heated working fluid 118 in the evaporator forces the vapor through vapor space 116 toward condenser section 124 of the heat pipe 100. Because condenser section 124 is at lower temperature than evaporator section 120 and the vaporization temperature of working fluid 118, the vapor condenses back into a liquid, giving up to heat sink 128 its latent heat of vaporization, which was acquired in evaporator section 120. The now again liquid phase working fluid 118 is absorbed by wick 114 in condenser section 124 and capillary action wicks the liquid back toward evaporator section 120 where it is again available for evaporation. This process rapidly reaches equilibrium and operates continuously as long as heat is supplied.
The type of working fluid 118 generally influences and limits the performance of heat pipe 100 in several ways. These are usually related to the “transport properties” of the working fluid 118, which is generally defined by a Figure of Merit known as the Liquid Transport Factor (M), given by the following equation:M=(ρL*σ*λ*cosθ)μL 
Where M=Liquid Transport Factor; ρL=Liquid Density; σ=Surface Tension; λ=Enthalpy of Vaporization, θ is the wetting angle, and μL=Liquid Viscosity. As known in the art, ρL, σ, and λ all decrease with increasing temperature (T), and μL increases with increasing T. Provided the working fluid is operable within the desired temperature range, the working fluid is often selected based on its Enthalpy of Vaporization (λ) in an attempt to maximize M and thus increase the heat transfer efficiency of the heat pipe or other heat transfer device. Another class of evaporating/condensing heat transfer device is the reflux boiler, which utilizes gravity rather than wick capillary pumping used by typical heat pipe 100 to return liquid from the condenser to the evaporator. Reflux boilers are also known as wickless heat pipes, or two-phase closed thermosyphons.